Particular embodiments generally relate to amplifiers.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In a read channel for a hard disk drive, a signal received from a read head of the disk drive may be asymmetric. For better performance, the symmetry of the asymmetric signal should be corrected.
FIG. 1a depicts a graph 100 of an asymmetric signal 102 and ideal signal 104. Ideal signal 104 includes pulses in a period that have absolute peak amplitudes that are equal and asymmetric signal 102 has pulses that have unequal absolute peak amplitudes in the period. The correction of asymmetric signal 102 is performed to correct the asymmetry of asymmetric signal to be similar to the symmetry of ideal signal 104.
One way of correcting asymmetric signal 102 is to generate a square term that increases or decreases the amplitude of asymmetric signal 102. FIG. 1b depicts a graph showing the correction. A square term 106 is combined with asymmetric signal 102 to produce an output signal 108. However, an additional path and extra circuitry are needed to generate the square term.
FIG. 2 depicts a conventional asymmetry correction circuit 200. The gain of an amplifier 202 is set by a resistance ratio for transistor M2 and transistor M1 (M2/M1). That is, the resistance of transistor M2 divided by the resistance of transistor M1 defines the gain. A gate voltage Vgate is tuned to determine the desired resistance for transistor M1 and transistor M2. For example, the resistance of transistors M1 and M2 may be varied based on the gate voltage applied to the transistors. Also, a gate voltage Vmrg is varied separately for transistors MR1 and MR2 to provide different magneto-resistive (MR) asymmetry.
The resistance of transistor M1 may vary based on a drain-source voltage across transistor M1. To limit the variation, gate voltage Vgate needs to be high enough to put transistor M1 into saturation or overdrive. Because the input voltage swing (i.e., the swing between voltages INP and INM) may be large, distortion may occur. To lower the distortion, voltage Vgate may need to be higher than a supply voltage to achieve low distortion. A charge pump may be needed to generate this voltage, which introduces additional circuitry. One way to lower the distortion is to separate transistor M1 into multiple transistors (such as two transistors in series). However, this increases the size of the transistor and introduces parasitic capacitance at a junction M located at the input of amplifier 202. This limits the bandwidth of amplifier 202 because the capacitance is at the second pole. Also, because the input impedance of transistor M1 varies with the input signal swing, if a source follower is driving transistor M1, the output resistance of the source follower needs to be the dominant factor requiring more current than is needed to be used at that stage.